In a famous scene from Alfred Hitchcock’s classic thriller Rear Window, injured photographer L.B. “Jeff” Jefferies tries to keep a murderer at bay by repeatedly blinding him with a camera flashbulb.

Whether or not the 1954 movie got them started, the U.S. Army once tried to turn the same basic idea into an actual weapon. During the Vietnam War, the U.S. Army’s Limited War Laboratory—a.k.a. LWL—in Maryland cooked up a so-called “ambush light” for American.

During a night-time ambush, the portable lamp would blind enemy forces and make them easier to see … and kill.

It might seem silly, but the project derived from the terrifying nature of infantry combat in Vietnam. The most terrifying form it took was the night-time ambush, where soldiers would spring a sudden attack on their enemies.

During such a surprise attack, ambushed soldiers could suffer extreme casualties in minutes or seconds. The almost pitch-black darkness added to the confusion and horror — even more frightening if the victims of the attack were caught in open terrain.

Thousands of Americans died in ambushes. But U.S. troops also became deadly professionals in the art, and deployed illumination to give themselves a killing edge.

“One of the essential requirements for a successful ambush is the element of complete surprise,” one LWL project report stated. “During night ambushes, success is enhanced if the surprise element is combined with simultaneous illumination.”

To see their enemies, American troops would shoot flares into the air, suddenly lighting up the battlefield. But this tactic would potentially revealtheir own positions — spoiling the ambush.

Other times, American troops hid trip-wire flares. When Viet Cong guerrillas tripped the wires, the sudden flash of light would signal to the Americans where to shoot. But those signals weren’t always accurate.

Hence LWL’s 1968 prototype “ambush light.”

Really a giant lamp, the device produced a beam of light five degrees high and 40 degrees wide. The cone could completely light up a person 100 feet away. Powered by a dozen “D” batteries, the light would run non-stop for 15 minutes.

Troops could strap the whole setup to a tree or to the carrying case—which was a standard ammunition box. In the field, soldiers could daisy-chain multiple lights together, and turn the whole array on with the flip of a a single switch.

Two years later, LWL shipped 25 ambush lights to troops in Vietnam and two more to American forces in Korea. We don’t know what happened with these early prototypes, but the Army engineers quickly went back to the drawing board.

We don’t know the reason why. Perhaps troops found the light too dim or small to be useful. It’s possible that the lamps experienced electrical problems. Southeast Asia’s brutal humidity had ruined the inner workings of other electrical gear, so it wouldn’t have been unprecedented.

Whatever the case, the LWL decided to replace the battery-powered lamp with a more traditional flare. To build this new light source, the Army first looked to Thiokol Chemical Corporation. Known best for building rocket motors, Thiokol was already developing a small, brightly burning light source for a riot-control device called the “Bright Light Mob Dispersal.”

The “candle” in this over-powered flashlight burned at 300,000 candlepower—some three times as bright as direct sunlight at high noon.

But in a series of comparison tests, engineers discovered these expensive purpose-built devices weren’t necessarily better than traditional signal flares. So engineers simply strapped the innards from two M-127A1 signal flares together.

The combined magnesium-based filler only burned for one minute and generated some 125,000 candlepower. Though less powerful than Tiokol’s design, the lamp effectively produced a cone of artificial daylight.

The final prototype consisted of this flammable core along with a lightweight reflective shield. A soldier would trigger the light with the same firing device that came with the M-18 Claymore landmine. LWL stuck all of this gear inside a special waterproof case rather than an ammunition container.

All told, the whole kit weighed four and a half pounds.

While certainly functional, the engineers quickly discovered that their new design wasn’t very rugged. During one trial, testers accidentally broke a third of the kits just by manhandling them over a series of obstacles. Another experiment showed that the flares could pose a fire hazard if hit during a firefight.

But the Army didn’t feel these issues were serious enough to keep the prototypes out of the field. The gear was reliable enough for field duty, and commanders would simply warn their soldiers that the packs could burst into flames.

A year after redesigning the entire device, the Army shipped 200 of these new lamps to Vietnam. Despite a positive reception in Vietnam, the brass back in the states decided there wasn’t any need for the ambush lights.

For the next 14 months, the project wallowed in administrative limbo. Then in December 1972, the Army told the LWL to get back to work.

It turns out — commanders in Southeast Asia still wanted their ambush lights. Unfortunately, “certain unresolved problems limit[ed] its reliability, especially at the temperature extremes,” the LWL’s final report on the project lamented.

The reliability issues only became more pronounced in Vietnam’s damp environment. After six years, the engineers still complained that they needed more time to perfect the system.

“During this program, various changes were incorporated by necessity into the ignition mechanism … and due to insufficient time it was not developed to a more advanced state,” the LWL’s final review stated.

By that time, America’s war in Vietnam was finished — as were further increases to the Army’s budget.

With no money left, and in a last ditch effort to keep up interest, the LWL sent off 60 prototypes to Rangers at the Army’s Infantry School and another 20 to Special Forces troops at the Institute for Military Assistance.

It wasn’t enough to save the project, and the ambush light quickly faded into obscurity. Eventually, night vision gear made the whole concept obsolete. By turning up ambient light, soldiers could get the drop on their opponents in the dark without giving away their position—and for more than just 60 short seconds.

Today, U.S. troops do carry lasers that can temporarily blind and disorient people. However, these beams are meant to stop suicide bombers at checkpoints or help quell riots rather than kick off ambushes.